Metal working or forming operations require the use of a lubricant in order to reduce the forces between the part to be worked and the tool, to remove the chips and fragments, to cool the part or the plate worked, and to control its surface texture. Oil-based lubricants have conventionally been used. These are whole oils or emulsions to which lubricity promoters, antiwear (AW) and/or extreme pressure (EP) additives may have been added. EP additives are generally compounds containing sulfur. At the high temperatures prevailing at the contact between the metal parts during metal working operations, the sulfur compounds decompose. A layer of iron sulfide is formed on the surface of the parts, hindering the processes of welding and adhesion.
Whole oils have excellent lubricating properties, but when production rates are high, the removal of the heat generated requires the use of emulsions. However, the use of emulsions also tends to be limited because, over time, they deteriorate and give off foul odors. This is why the use of aqueous fluids is steadily spreading. These are either synthetic fluids, which are aqueous solutions based on water-soluble additives, or semisynthetic fluids which are oil-in-water microemulsions containing a large quantity of emulsifiers. However, while the aqueous fluids effectively remove the heat and display improved resistance to bacterial proliferation, they are often limited to metal working operations in which the friction and wear conditions are not too severe. This is because EP additives have been developed specifically for oils, so that very few of these additives are water-soluble and therefore suitable for aqueous fluids.
While numerous oil-soluble EP additives are available, the number of water-soluble EP additives is much smaller. In Lub. Eng. 1977, 3(6), 291-298, R. W. Mould et al. describe the EP properties of a number of water-soluble sulfur-bearing additives, such as sodium salts of thiosalicylic, 2-mercaptopropionic, 2,2′-dithiodibenzoic, 2,2′-dithiodipropionic and dithiodiglycolic acids. Similarly, the use of water-soluble salts of 3,3′-dithiodipropionic acid has been the subject of U.S. Pat. No. 4,880,552 and JP 63 265 997. In U.S. Pat. No. 4,606,833, dithiodiglycol is used in combination with a derivative of polyoxyalkylene glycol to improve the extreme pressure properties of aqueous lubricants. The use of derivatives of aminosulfonic acids as additives in aqueous lubricating systems has been the subject of patent application WO 8602941. More recently, in Japanese patent application JP 10 110 181, the water-soluble salts of 3,3′-dithiodipropionic acid were combined with alkyl thioacids in aqueous drawing and stamping lubricants.
However, these products are not sufficiently stable in water. They promote bacterial proliferation and liberate hydrogen sulfide, causing the aqueous fluids to emit a strong odor. Their formulation thus requires the use of a large quantity of bactericides, which is incompatible with the quantities usually permitted in aqueous lubricants. Furthermore, some of these extreme pressure additives are incompatible with most of the other additives routinely used in synthetic and semisynthetic formulations.
Recently, lubricant compositions containing salts of sulfamic acid amines with extreme pressure effect have been the subject of patent application WO 00/44848. However, sulfamic acid is highly corrosive. Moreover, its low solubility precludes the production of liquid concentrates that are easy to use in the formulations. Furthermore, to limit the environmental impact of the lubricant formulations used in the field of metal working and forming, it is particularly important to employ readily biodegradable additives.